Although we never learn much about Jack Starbright, her presence emphasizes Alex's need for grown-up guidance. Not only has Alex lost his uncle—who was like a father to him—but he also has to face the possibility of losing his home and, more importantly, losing Jack.
Jack has lived with Alex and Ian for so long that she's practically family, but "her visa is about to expire" (3.9). This comes in the midst of the chaos surrounding Ian's death, which has already placed Alex's future in a good deal of jeopardy. Ian and Jack are Alex's only family, and now this fourteen-year-old is looking at a future as an orphan—again.
Ironically, Alex must go on a very-adult spy mission so he can hold on to his childhood. Despite everything, Alex is still a fourteen year-old boy in all of the ways that matter, so he's lucky to have the moral support of someone like Jack in his life.